Re: [PATCH v2 1/3] xen-blkback: add a parameter for disabling of persistent grants

[SeongJae Park <sjpark@amazon.com>]

On Tue, 22 Sep 2020 13:12:59 +0200 "Roger Pau Monné" <roger.pau@citrix.com> wrote:

> On Tue, Sep 22, 2020 at 12:52:07PM +0200, SeongJae Park wrote:
> > From: SeongJae Park <sjpark@amazon.de>
> > 
> > Persistent grants feature provides high scalability.  On some small
> > systems, however, it could incur data copy overheads[1] and thus it is
> > required to be disabled.  But, there is no option to disable it.  For
> > the reason, this commit adds a module parameter for disabling of the
> > feature.
> > 
> > [1] https://wiki.xen.org/wiki/Xen_4.3_Block_Protocol_Scalability
> > 
> > Signed-off-by: Anthony Liguori <aliguori@amazon.com>
> > Signed-off-by: SeongJae Park <sjpark@amazon.de>
> > ---
> >  .../ABI/testing/sysfs-driver-xen-blkback      |  9 ++++++
> >  drivers/block/xen-blkback/xenbus.c            | 28 ++++++++++++++-----
> >  2 files changed, 30 insertions(+), 7 deletions(-)
> > 
> > diff --git a/Documentation/ABI/testing/sysfs-driver-xen-blkback b/Documentation/ABI/testing/sysfs-driver-xen-blkback
> > index ecb7942ff146..ac2947b98950 100644
> > --- a/Documentation/ABI/testing/sysfs-driver-xen-blkback
> > +++ b/Documentation/ABI/testing/sysfs-driver-xen-blkback
> > @@ -35,3 +35,12 @@ Description:
> >                  controls the duration in milliseconds that blkback will not
> >                  cache any page not backed by a grant mapping.
> >                  The default is 10ms.
> > +
> > +What:           /sys/module/xen_blkback/parameters/feature_persistent
> > +Date:           September 2020
> > +KernelVersion:  5.10
> > +Contact:        SeongJae Park <sjpark@amazon.de>
> > +Description:
> > +                Whether to enable the persistent grants feature or not.  Note
> > +                that this option only takes effect on newly created backends.
> > +                The default is Y (enable).
> > diff --git a/drivers/block/xen-blkback/xenbus.c b/drivers/block/xen-blkback/xenbus.c
> > index b9aa5d1ac10b..8a95ddd08b13 100644
> > --- a/drivers/block/xen-blkback/xenbus.c
> > +++ b/drivers/block/xen-blkback/xenbus.c
> > @@ -879,6 +879,12 @@ static void reclaim_memory(struct xenbus_device *dev)
> >  
> >  /* ** Connection ** */
> >  
> > +/* Enable the persistent grants feature. */
> > +static bool feature_persistent = true;
> > +module_param(feature_persistent, bool, 0644);
> > +MODULE_PARM_DESC(feature_persistent,
> > +		"Enables the persistent grants feature");
> > +
> >  /*
> >   * Write the physical details regarding the block device to the store, and
> >   * switch to Connected state.
> > @@ -906,11 +912,15 @@ static void connect(struct backend_info *be)
> >  
> >  	xen_blkbk_barrier(xbt, be, be->blkif->vbd.flush_support);
> >  
> > -	err = xenbus_printf(xbt, dev->nodename, "feature-persistent", "%u", 1);
> > -	if (err) {
> > -		xenbus_dev_fatal(dev, err, "writing %s/feature-persistent",
> > -				 dev->nodename);
> > -		goto abort;
> > +	if (feature_persistent) {
> > +		err = xenbus_printf(xbt, dev->nodename, "feature-persistent",
> > +				"%u", feature_persistent);
> > +		if (err) {
> > +			xenbus_dev_fatal(dev, err,
> > +					"writing %s/feature-persistent",
> > +					dev->nodename);
> > +			goto abort;
> > +		}
> >  	}
> >  
> >  	err = xenbus_printf(xbt, dev->nodename, "sectors", "%llu",
> > @@ -1093,8 +1103,12 @@ static int connect_ring(struct backend_info *be)
> >  		xenbus_dev_fatal(dev, err, "unknown fe protocol %s", protocol);
> >  		return -ENOSYS;
> >  	}
> > -	pers_grants = xenbus_read_unsigned(dev->otherend, "feature-persistent",
> > -					   0);
> > +	if (feature_persistent)
> > +		pers_grants = xenbus_read_unsigned(dev->otherend,
> > +				"feature-persistent", 0);
> > +	else
> > +		pers_grants = 0;
> > +
> 
> Sorry for not realizing earlier, but looking at it again I think you
> need to cache the value of feature_persistent when it's first used in
> the blkback state data, so that it's consistent.
> 
> What would happen for example with the following flow (assume a
> persistent grants enabled frontend):
> 
> feature_persistent = false
> 
> connect(...)
> feature-persistent is not written to xenstore
> 
> User changes feature_persistent = true
> 
> connect_ring(...)
> pers_grants = true, because feature-persistent is set unconditionally
> by the frontend and feature_persistent variable is now true.
> 
> Then blkback will try to use persistent grants and the whole
> connection will malfunction because the frontend won't.

Ah, you're right.  I should also catch this before but didn't, sorry.

> 
> The other option is to prevent changing the variable when there are
> blkback instances already running.

I think storing the option value in xenstore would be simpler.  That said, if
you prefer this way, please let me know.


Thanks,
SeongJae Park